In recent years, with the spread of reconfigurable optical add/drop multiplexers (ROADM), availability of a network connection has increased. For this reason, there is an increasing demand for a function that may observe a network connection status in a simple manner. As one example of a technology that meets such a demand, a technology has been known that superimposes a control signal such as a supervisory signal onto an optical signal that carries data.
For example, in a WDM transmission system, the control signal for identifying an optical path is superimposed onto the optical signal (a main optical signal) of each wavelength (channel), which is included in a WDM signal. In this case, the optical transmission apparatus (hereinafter referred to as a “node”) or a receiver may recognize a transmission source of the optical signal and a path of the optical signal by demodulating the control signal from the received optical signal.
In such a light transmitting system, data on each channel is modulated, for example, in a multi-level modulation manner such as QPSK, mQAM (m is 16, 64, and 256) and thus is transmitted along an optical transmission path. On the other hand, the control signal such as the supervisory signal, for example, is superimposed on main signal light in a frequency shift keying (FSK) manner.
Moreover, although an object of the disclosure is different, International Publication Pamphlet No. WO 2010/026757 discusses that a transmission apparatus phase-modulates or frequency-modulates an optical signal, which is phase-modulated based on a first signal, based on a second signal that is slower in speed than the first signal, and thus transmits the phase- or frequency-modulated optical signal. Furthermore, a receiving apparatus disclosed in International Publication Pamphlet No. WO 2010/026757 splits the optical signal received from a transmitting apparatus, performs delaying interference on one split optical signal and thus outputs interference light. Then, the receiving apparatus converts the interference light into an electric signal, and thus obtains the second signal based on the full-wave rectified signal and obtains the first signal based on the other split signal. Accordingly, in the technology disclosed in International Publication Pamphlet No. WO 2010/026757, multiple signals that are different in speed may be multiplexed, and information may be multiplexed in the shape of a single peak spectrum in a single wavelength slot.
Furthermore, Japanese Laid-open Patent Publication No. 02-096719 discusses an optical signal demodulation circuit including an optical filter that allows a specific optical frequency for the optical signal that is frequency-modulated to an n value (n is an integer that is 2 or greater) to pass through and a light receiving element that receives light that passes through the optical filter. The optical filter disclosed in Japanese Laid-open Patent Publication No. 02-096719 is a Mach-Zehnder type periodic optical filter that outputs optical frequencies corresponding to the n values to different output ports.
If the control signal is superimposed onto the optical signal in a frequency shift keying manner, the receiver, for example, demodulates the control signal by converting a frequency modulation component of the optical signal into an amplitude component. However, in this case, when the frequency modulation component for superimposing the control signal onto the optical signal is large (that is, when a range of frequency change is wide), there is a concern that quality of data (for example, an error rate) that is carried by that optical signal will deteriorate. For this reason, it is preferable that the frequency modulation component for superimposing the control signal onto the optical signal be small enough for the quality of the data not to deteriorate.
When the frequency modulation component of the optical signal is small, the amplitude component that is obtained from the frequency modulation component is also small in the receiver. In such a case, the amplitude component that is obtained from the frequency modulation component of the optical signal is easily influenced by an intensity modulation noise (an AM noise) that occurs in a transmission path between the transmitter and the receiver and the like. Therefore, detection sensitivity of the control signal is decreased in the receiver.
Moreover, the detection sensitivity of the frequency modulation signal that is superimposed onto the optical signal is not disclosed in International Publication Pamphlet No. WO 2010/026757 and Japanese Laid-open Patent Publication No. 02-096719.